1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and more particularly to an image forming apparatus having a re-feeding unit for feeding sheet with an image formed on one side to a transfer unit to form another image on the sheet.
2. Description of the Prior Art
To meet the recent requirement for multifunction of a copying machine, there has recently been used a copying machine having a composite copying function capable of effecting transfer of images a plural number of times onto one side of a single sheet, or a copying machine having a both-side copying function capable of effecting transfer of images onto both the surface and the back. Further, in U.S. Ser. No. 883,144 filed on July 8, 1986 there is disclosed a copying machine having both a composite copying function of making transfer of images in plural number of times onto one side of sheet and a both-side copying function of making transfer of images onto both sides of sheet.
A copying machine having any of the above-mentioned functions is provided with a primary passage for feeding each sheet from the interior of a feeding section which contains copying sheets up to a discharge section through a transfer section and a fixing section and also provided with a secondary passage contiguous to the primary passage and formed in a re-feeding section which is for returning the sheet with an image formed thereon past the fixing section again to the transfer section.
Since the sheet fed into the secondary passage has once passed through the fixing section, it is in a curled state with heat or its water content or stiffness has been changed as compared with the original sheet, resulting in that jam of sheet is more likely to occur in the secondary passage.
According to the prior art, in the event of a jam in the sheet feeding passage, the occurrence and position of the jam are detected by a sheet position detecting sensor and a timer which detects the time during which the sheet is detected by the sensor, and the jam position is indicated by an indicator. In order to remove the sheet which is causing the jam to the exterior of the copying machine, the sheet conveying path is capable of being opened. The operator puts his hand into the jam portion of the interior of the copying machine and removes the sheet to the exterior.
Consequently, a conventional copying machine of this type becomes larger in size in order to ensure the working space of the operator. Particularly, recent copying machines are compact, so in the case where a secondary passage for effecting re-feed is provided, there is no extra space for opening the said passage in the interior of the machine, resulting in that it is becoming more and more difficult to deal with the jam of sheet in the secondary passage.